1. Field of the Invention
This invention relates to a process for covering the surface of a lens preform with a plastic sheet in which the plastic sheet is applied to the glass preform using an elastic pressing element while eliminating the air therebetween. Optionally, an additional step can be performed to obtain the desired final adherence between the plastic sheet and the glass preform, by applying much higher pressure and temperature conditions.
2. Description of the Prior Art
Processes of this type are used when it is desired to cover protective glass or lenses with a sheet of transparent plastic which is used as an antishattering sheet. For example, German Patent Publication DE AS 26 29 779 describes these anti-shattering sheets which are preferably formed of high elasticity polyurethane having self-recovery properties.
These properties give the sheet a better resistance to scratches which then disappear very quickly. These sheets can also be used for producing plastic glass which is scratch-resistant. The German Patent Publication DE OS 26 34 816 describes, for example, plastic glass thus coated.
The application of these protective sheets to silicate glass or organic (plastic) glass poses certain problems when the surface to be covered is concave. In the first place, there is a risk of a crinkling of the sheet due to the fact that the sheet, flat in itself, must be adapted to a curved surface. Moreover, when convex spherical surfaces are covered, channels are formed, in a direction radial to the pressing element, and in which the displaced air preferably flows out, which causes a poor adherence of the sheet.
Because of specific problems which arise when it is desired to cover spherical surfaces, it has been found that processes in several stages are particularly advantageous. Preferably, a process in three stages is used. French Patent Application No. 82 12 542 describes in detail a method for performing the first stage of the process, i.e., the positioning of a piece of sheet on the glass or plastic preform, and temporary holding of it in this position. At the end of this first stage, there results a preform, of glass or plastic, to which the sheet is attached so that the preform can be handled without the risk of detaching the sheet. Between the sheet and the surface of the preform there is still a relatively large amount of air which must be radially expelled in a second stage.